The present disclosure relates to a sensor unit which is incorporated in image forming apparatuses such as facsimile machines, copiers, and printers and which includes a sensor for sensing the edge of a recording medium in sheet form, and also relates to image forming apparatuses provided with such a sensor unit.
Image forming apparatuses such as facsimile machines, copiers, and printers are configured to record an image on a recording medium such as paper, cloth, and OHP sheets. These image forming apparatuses can be classified into an electrophotographic type, an ink-jet type, etc. based on the recording method.
During printing on a recording medium using an image forming apparatus, if the recording medium deviates in the direction (recording medium width direction) perpendicular to the conveying direction, the printing position on each recording medium deviates. Thus, for example in a case where binding is performed after printing, high accuracy in the printing position on each page is required. In particular, if an ink-jet recording apparatus is used, ink is prone to soak through to the rear face of the recording medium, and thus very high accuracy (for example, several tenths of a millimeter or less) in the printing position is required in double-sided printing.
To cope with that, in conventional image forming apparatuses, there is arranged a CIS (contact image sensor), facing a conveying belt for conveying a sheet (recording medium), for sensing the position of an end part of a sheet in its width direction. This image forming apparatus senses the position of an end part of a sheet in its width direction based on the difference in the intensity of the light received by the CIS with and without a sheet.
For example, a known image forming apparatus is provided with a deviation amount sensing means, an error sheet sensing means, a double-side path sheet sensing means, a conveyance source judging means, and a sheet discharging means. The deviation amount sensing means senses the lateral deviation amount and the skew amount of a sheet with a CIS. The error sheet sensing means, when the sensed deviation amount of a sheet is larger than a predetermined threshold value, judges the sheet to be an error sheet and senses its appearance. The double-side path sheet sensing means, when an error sheet appears, senses whether a sheet is present in the double side path. The conveyance source judging means, when a sheet is present in the double side path, judges whether the error sheet is a sheet conveyed from a sheet storing portion or a sheet conveyed through the double side path. The sheet discharging means determines the sheet to be discharged based on the judgement result and discharges the determined sheet.
A known edge sensing device binarizes the output value of a CIS arranged in a conveying passage of a conveyed object (sheet), and when the position at which the binarized value changes is within an edge sensing range which is stored for each conveyed object size, judges the position to be the edge position of the conveyed object. Also known is shifting a conveyed object in the width direction based on the deviation amount between a sensed edge position and a reference position.